1. Field to Which the Invention
The present invention relates to an output shaft structure for a shaft drive vehicle such as a four-wheel drive vehicle, and particularly to an output shaft structure capable of easily performing a change between power supplied to both the front and rear wheel drives and power supplied to the rear wheel drive.
2. Description of Background Art
Japanese Patent Laid-open No. Hei 8-25979 discloses a four-wheel buggy on which an engine with its crank shaft directed in the longitudinal direction of the vehicle is mounted (the engine of this type is hereinafter referred to as a xe2x80x9clongitudinally mounted enginexe2x80x9d), wherein four front and rear wheels are shaftdriven. The four-wheel buggy has a power transmission structure in which an output shaft is provided in such a manner as to pass through a lower portion of a power unit case in the longitudinal direction, and the front end, projecting forwardly from the power unit, of the output shaft is connected to a front wheel side propeller shaft to drive the front wheel and the rear end, projecting rearwardly from the power unit, of output shaft is connected to a rear wheel side propeller shaft to drive the rear wheel.
The above structure, however, has the following inconvenience: namely, since the output shaft passes through the power unit case, the output shaft and the power unit case for the four-wheel buggy having the front and rear wheel drive specification cannot be commonly used for a vehicle having a rear wheel drive specification. In other words, output shafts and power unit cases specialized for the rear wheel drive specification and the front and rear wheel drive specification must be individually manufactured. In view of the foregoing, a primary object of the present invention is to realize the common use of an output shaft between vehicles having a front and rear drive specification and a rear wheel drive specification.
Since the conventional output shaft is formed of a single shaft member extending over the entire length, it must be often supported by bearing portions disposed at three or more points. If the output shaft is supported by three bearings, the assemble of the output shaft with the three bearings becomes impossible unless the journal diameter of each of the bearing portions at both the ends is smaller than the inside diameter of the central bearing. Accordingly, the size of the bearings at both the ends exert an effect on the size of the central bearing. On the contrary, if the size of the bearings at both the ends are set at required values, the inside diameter of the central bearing must be set at a larger value. This results in a dimensional limitation in which distances between the output shaft and the adjacent shaft and its peripheral parts must be made larger.
As a result, according to the conventional single longer output shaft, it may be difficult to achieve the requirement to make the power unit compact. The longer single output shaft has another problem that since the longer output shaft requires a plurality of bearing portions whose dimensions must be accurately kept for preventing the runout thereof, the formation of the bearing portions must be accurately controlled at the manufacturing steps. The longer the output shaft becomes, the more the keeping of accuracy becomes difficult. Also, to obtain such high accuracy bearing portions, the manufacture of the output shaft requires a large number of steps, such as preparation of the base material, rough-machining of the bearing portions, heat-treatment, bend flattening, polishing of the bearing portions, and re-straightening. In view of the foregoing, a secondary object of the present invention is to solve the above-described problems.
To achieve the above objects, according to the present invention, there is provided an output shaft structure for a shaft drive vehicle including an output shaft for transmitting the output of an engine to both the front wheel side and the rear wheel side, characterized in that the output shaft is connectably divided into a front wheel side shaft portion and a rear wheel side shaft portion.
According to the present invention, the front wheel side shaft portion is spline-connected to the rear wheel side shaft portion.
According to the present invention, the output shaft is divided into the front wheel side shaft portion and the rear wheel side shaft portion, and accordingly, in the case of applying the output shaft structure to the front and rear wheel drive specification, the front wheels and the rear wheels can be simultaneously driven by using the output shaft composed of the front wheel side shaft portion and the rear wheel side shaft portion connected to each other, and in the case of applying the output shaft structure to the rear wheel drive specification, only the rear wheels can be driven by using the output shaft composed of only the rear wheel side shaft portion to which the front wheel side shaft portion is not connected. In this way, according to the present invention, the rear wheel side shaft portion can be commonly used for the rear wheel drive specification and the front and rear wheel drive specification, and the power unit case can be also commonly used for both the specifications except that the power unit case is slightly machined for the front and rear wheel drive specification. As a result, it is possible to easily perform the change between the front and rear wheel drive specification and the rear wheel drive specification.
According to the present invention, since the front wheel side shaft portion is spline-connected to the rear wheel side shaft portion, both the shafts can be easily connected to and separated from each other. Also, since the outer periphery of a portion, located rearwardly from the male spline (that is, the connection portion to the front wheel side shaft portion), of the rear wheel side shaft portion is rotatably supported, the journal diameter of the bearing portion can be made small. As a result, it is possible to shorten a distance between the output shaft and the transmission side, and hence to make the entire power unit compact. Further, since the output shaft is divided into parts, it is possible to easily manufacture the output shaft because the degree of control of accuracy of each of the parts at the manufacturing steps can be made moderate.